This invention relates to voltage magnitude control systems and particularly to those of the tap changing type.
A step voltage regulator is an autotransformer provided with load ratio control equipment for regulating the voltage on the feeder or bus to which it is connected. A typical step voltage regulator may have a 100 percent exciting winding in shunt with the line on the source side and normally maintains the voltage on the load side within a desired voltage bandwidth by a 10 percent tapped buck/boost winding in series with the line. The series winding has taps connected to the stationary contacts of a tap changer dial switch having a pair of rotatable selector contacts driven by a reversible motor into sequential engagement with the stationary contacts and usually provides the ability to change the effective turns ratio from input to output plus and minus 10 percent in 32 steps of 5/8 percent voltage increments. The rotatable contacts of the tap changer switch are usually connected through slip rings to the opposite ends of a bridging center-tapped autotransformer reactor, termed a preventive autotransformer, to permit transition from one tap position to another without interrupting the load circuit. When the selector contacts bridge adjacent stationary contacts, the high reactance of the preventive autotransformer limits circulating current to a safe value and thus reduces burning and erosion of the tap changer contacts and also provides a voltage midway between the physical taps to thereby provide twice the number of steps. However, such a preventive autotransformer has continuous energy losses in operation, is bulky, and is expensive to construct. Further, substantial arcing occurs at the tap changer dial switch contacts of a regulator employing a preventive autotransformer due to inductive energy storage even when the selector contacts are operated with quick-break action during each tap change, and such arcing necessitates the use of expensive arc resistant material for the contacts and also contaminates the oil in which the tap changer switch is immersed.